No I didn't remove the previous version, perhaps this was the cause, I will try this later on. For the avoidance of doubt, its the new Maxwell 4 plugin that I was trying to use with the latest version of Z. (El Capitan)

The issue is that GPU support is only nVidia CUDA based cards and they are generally not available on Macs. In fact all current Macs that are shipping support use AMD cards NOT nVidia and they are will not work like all other GPU renderers.

It is becoming more and more apparent that Windows based machines are replacing Mac's for 3D work as Apple is not keeping pace with high end GPU support and persists with just AMD /Intel graphics support that just does not cut it - much to my displeasure!

The new Maxwell 4 packages are are modeller specific and only work with the modeller you get it for - but the upside is formZ plugin is in the less expensive lot.

well, other engines are supporting mac GPU rendering, like thea (which you will find as a plugin for FZ) and octane... the problem is that next limit jumped late on the GPU bandwagon, so they have not much experience yet with coding this type of stuff - which needs higly specialized algorithms and a good knowledge of how to use the CUDA platform. waht you get now is basically an early alpha stage of that engine, which will be refined 'on the go'. also, with the new version, they changed their marketing and abandon a strategy which was their big advantage: in the past you bought a maxwell licence and got plugins for all supported platforms. now you have to pay separately for everything, a way to make more money on the paper, but i doubt they will make more clients like this. also, they abandoned a few 'obsolete' platforms like lightwave(which is definitely not obsolete), houdini and softimage.

the FZ plugin is definitely well done and updated with care, i've used it in the past. so if you don't expect GPU rendering to be production ready within the next months, it's worth to be considered as a third party engine for FZ. i also tried out the thea plugin lately and liked the integration a lot. but development seems to be stalled and issues like palettes not appearing and a few instabilities are holding me back ftom buying a license and there has not been any update for a few months now. hopefully, the developer will care more about fixing issues, as it is something i'm looking into for using my GPU capacities on mac in the future.